The present invention relates of magnetic resonance imaging (MRI) coils and, in particular, to array coils therefor.
The phased array configuration for RF coils in magnetic resonance imaging was introduced by Roemer et al. as set forth in U.S. Pat. No. 4,825,162. Based on this design a set of surface coil loops are critically decoupled relative to each other and simultaneously receive the NMR signal which is generated in the imaging region that is excited by a dedicated volume coil. The signal from these surface coils is then reconstructed to generate an image which is multiple times larger that the size of each individual loop in the array. According to this method, because the size of each element in the phased array is relatively small compared with the final reconstructed image, the noise figure is reduced and the signal obtained from each element of the phased array is increased, and the final results is a significant increase in Signal-to-Noise Ratio (SNR) for the final reconstructed image. The first application of this phased array design was intended for the spine region. The logic behind this application was that the required coverage of the spine area on the human body is extremely large and a single receive coil structure can not adequately provide the image quality necessary for clinical evaluation.
Modification have been presented over the years including modifications or improvement of Roemer's invention. Specifically Keren. in U.S. Pat. No. 5,198,768 extended Roemer's invention to include figure-8 shaped loops commonly known as “butterflies”.
Following that patent was U.S. Pat. No. 5,543,711 by Srinivasan et al. where instead of surface coils for receive only purposes, he only utilized birdcage resonators which are volume RF coils for a phased array configuration.
In order to enhance the ability to image the cervical-thoracic-lumbar spine area, a new phased array design (U.S. Pat. No. 6,624,633) was developed to cover this area. This design includes, for example, 6 quadrature pairs (6 loop and 6 saddle coils in quadrature configuration).
In U.S. Pat. No. 5,910,728 entitled “Simultaneous acquisition of Spatial Harmonics (SMASH): Ultra Fast Imaging with Radio Frequency Coils”, Sodickson describes a parallel processing algorithm that exploits spatial information inherent in a surface coil array to increase magnetic resonance (MR) image acquisition speed, resolution and/or field of view. Partial signals are acquired simultaneously in the component coils of the array and formed into two or more signals corresponding to orthogonal spatial representations. In a Fourier embodiment, lines of the k-space matrix required for image production are formed using a set of separate, preferably linear combinations of the component coil signals to substitute for spatial modulations normally produced by phase encoding gradients. The signal combining may proceed in a parallel or flow-through fashion, or as post-processing, which in either case reduces the need for time-consuming gradient switching and expensive higher field magnet arrangements.
In the post-processing approach, stored signals are combined after the fact to yield the full data matrix. In the flow-through approach, a plug-in unit consisting of a coil array with an on board processor outputs two or more sets of combined spatial signals for each spin conditioning cycle, each directly corresponding to a distinct line in k-space. This partially parallel imaging strategy, SMASH, is readily integrated with many existing fast imaging sequences, yielding multiplicative time savings without a significant sacrifice in spatial resolution or signal-to-noise ratio.
In a similar fashion another parallel processing algorithm was presented where the acceleration of image acquisition was performed on the time domain space instead of the frequency domain space. This parallel acquisition technique referred as SENSE (SENSitivity Encoding) by K. P. Prueessmann et. al. in their PCT Patent Application No. WO9954746A1 entitled “Magnetic Resonance Imaging Method and Apparatus” shows a method for obtaining images by means of magnetic resonance (MR) of an object placed in a static magnetic field, which method includes simultaneous measurement of a number of sets of MR signals by application gradients and an array of receiver coils, reconstruction of a number of receiver coil images from the sets MR signals measured and reconstruction of a final image from a distant dependent sensitivity of the receiver coils and the first plurality of receiver coil images. In order to reduce the acquisition time the number of phase encoding steps corresponding to the phase-encoding gradient is reduced with a reduction factor compared to standard Fourier imaging, while a same field of view is maintained as in standard Fourier imaging. In this way fast cardiac imaging may be possible. According to the application the calculation of complicated matrix inversion can be simplified by determining of an image vector of the final image from a combination of a generalized inverse of a sensitivity matrix and a receiver coil image vector. In this way aliasing artifacts in the final image are reduced. Furthermore, the reconstruction method enables application of non-integer reduction factors.
Over the past few years, different modifications of these techniques have been presented. GRAPPA (Generalized Autocalibrating Partially Parallel Acquisitions) technique (M. K. Griswold et. al. Magnetic Resonance in Medicine), a modified SMASH technique, where according to that technique, no detailed and highly accurate RF field map of the coil array is required prior to image acquisition. According to that technique, for each image acquisition, additional lines on the center of k-space are obtained that are needed to describe the RF field map of the array. Thus this technique can compensate for any motions or changes on the profile of the RF field map during the image acquisition. According to this technique, SNR and image quality are improved since the steps of image reconstruction and image combination are performed in separate steps.
The characteristics of all of these parallel imaging techniques is that the acceleration speed is directly proportional to the number of independent receivers along the direction that the image acceleration needs to be applied. In these terms, the higher the number of receiver coils the faster the acceleration speed for acquiring an image with better SNR and improved image quality. It is characteristic that for coils focused on SENSE applications the SNR of the accelerated image is given by:       SNR    SENSE    =      SNR          g      ⁢              R            where SNRSENSE represents the resulted Signal to Noise Ratio (SNR) of the SENSE image after the acceleration speed has been determined, SNR corresponds to the original SNR of the coil without SENSE acquisition and g defines the geometry factor of the coil that is strongly dependent on the geometric placement on the coil's mechanical former and their placement around the targeted field of view (FOV). The g value is a representation of the additional noise factor that the accelerated reconstructed image will be accompanied with. The optimum value of a g factor is 1 (no additional noise added during reconstruction) and a typical value for g value for clinically viable images ranges from 1.1 to 1.8 inside the designated FOV. M. Wieger in his paper entitled “Specific Coil Design for SENSE: A Six Element Cardiac Array” (Magnetic Resonance in Medicine, Vol. 45, 495–504 (2001).) indicates that the g factor for the coil significantly improves (closer to 1) when the coils on the array are not overlapping. He further indicates that the best g factor q achievable is when the coils are separated by a distance of 1 cm. The reason is that SENSE reconstruction software relies strongly on both the magnitude and phase information for each loop of the array coil system. By physically separating the coils in the array by a certain distance, the B1 field mapping (magnitude and phase) for each coil on the array can be uniquely defined in such a way that the reconstructed accelerated image contains little or not additional noise.
After the introduction of SMASH and SENSE techniques, a plethora or new coil designs were presented trying to incorporate the advantages of high speed imaging using parallel acquisition techniques. Wieger et al. used a six element array of loops separated by 1 cm between adjacent elements that was capable of accelerating the image acquisition by a factor of R=3 without degrading clinically the resulting images. This coil focused on imaging the periphery of the human torso and was concentrated only on the cardiac anatomy of the human body. Most of the SENSE, SMASH application papers are targeting brain imaging applications (“Design of a SENSE optimized High Sensitivity MRI Receive coil for Brian Imaging”, Jaco. A de Zwart, et. al. Mag. Res. Med. 47:1218–1227 (2002)). There has been no use of the techniques for imaging the cervical through the thoracic and ending on the lumbar spine.
U.S. Pat. No. 6,624,633 shows a cervical, thoracic, lumbar spine coil. The presence of a loop and a saddle coil provides the optimum SNR at the region of the spinal cord and the disc area, but it can not be altered to be suitable for SENSE and SMASH parallel processing applications. The reason is that the saddle and the loop are designed to provide the strongest and more uniform circularly polarized field on the spine area. since the B1 profile for these two coils is nearly indistinguishable around the spine region, the g-factor values for such a coil are severely elevated.